The web page adds this description:
Explanation: What are these humans doing? Dancing. Many humans on Earth exhibit periods of happiness, and one method of displaying happiness is dancing. Happiness and dancing transcend political boundaries and occur in practically every human society. Above, Matt Harding traveled through many nations on Earth, started dancing, and filmed the result.
And, as proof that these guys probably have the coolest jobs in the universe, there's this teaser at the bottom of the page:
Tomorrow's picture: martian cliff
(Hat tip to Gary.)